Lithium-containing complex oxide, non-aqueous secondary battery using the lithium-containing complex oxide, and method for producing the lithium-containing complex oxide

ABSTRACT

Because of the composition represented by General Formula: Li 1+x+α Ni (1−x−y+δ)/2 Mn (1−x−y−δ)/2 M y O 2  (where 0≦x≦0.05, −0.05≦x+α≦0.05, 0≦y≦0.4; −0.1≦δ≦0.1 (when 0≦y≦0.2) or −0.24≦δ≦0.24 (when 0.2&lt;y≦0.4); and M is at least one element selected from the group consisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge and Sn), a high-density lithium-containing complex oxide with high stability of a layered crystal structure and excellent reversibility of charging/discharging can be provided, and a high-capacity non-aqueous secondary battery excellent in durability is realized by using such an oxide for a positive electrode.

TECHNICAL FIELD

The present invention relates to a lithium-containing complex oxide thatcan be used as a material for a positive electrode of a non-aqueoussecondary battery, a non-aqueous secondary battery using thelithium-containing complex oxide, and a method for producing thelithium-containing complex oxide.

BACKGROUND ART

In recent years, along with the development of portable electronicequipment such as mobile phones and notebook computers, and thecommercialization of electric automobiles, there is an increasing demandfor a miniaturized and lightweight secondary battery with a highcapacity. At present, as a secondary battery with a high capacitysatisfying this demand, a non-aqueous secondary battery such as alithium secondary battery using LiCoO₂ for a positive electrode and acarbon material for a negative electrode is being commercialized. Such alithium secondary battery has a high energy density, and can beminiaturized and reduced in weight, so that it has been paid attentionto as a power source of portable electronic equipment.

LiCoO₂ used as a material for a positive electrode of the lithiumsecondary battery is easy to produce and handle, so that it is oftenused as a preferable active material. However, LiCoO₂ is produced usingcobalt, which is rare metal, as a material. Therefore, it is conceivablethat a material shortage will become serious in the future. Furthermore,the price of cobalt itself is high and fluctuates greatly, so that it isdesired to develop a material for a positive electrode that can besupplied stably at a low cost.

In view of the above, materials of a lithium-manganese oxide type areexpected to be a prospective substitute for LiCoO₂ as a material for apositive electrode of a lithium secondary battery. Among them,lithium-manganese oxides with a Spinel structure, such as Li₂Mn₄O₉,Li₄Mn₅O₁₂, and LiMn₂O₄, are being investigated. In particular, LiMn₂O₄can be charged/discharged in a potential range in the vicinity of 4 Vagainst Li metal. The use of LiMn₂O₄ is disclosed in at least thefollowing (JP 6 (1994)-76824 A, JP 7 (1995)-73883 A, JP 7 (1995)-230802A, JP 7 (1995)-245106 A, etc.).

The theoretical discharge capacity of LiCoO₂ is 274 mAh/g. However, whendeep charging/discharging is conducted, LiCoO₂ is changed in phase toinfluence a cycle life. Therefore, in an actual lithium secondarybattery, the practical discharge capacity falls in a range of 125 to 140mAh/g.

In contrast, the theoretical discharge capacity of LiMn₂O₄ is 148 mAh/g.However, LiMn₂O₄ also is changed in phase during charging/discharging inthe same way as in LiCoO₂. Furthermore, in the case of using a carbonmaterial as a negative active material, since the irreversible capacityof the carbon material is large, the discharge capacity that can be usedin the case where LiMn₂O₄ is used actually for a battery is decreased toabout 90 to 105 mAh/g. As is apparent from this, when LiMn₂O₄ is used asa positive active material, the battery capacity cannot be increasedcompared with the case where LiCoO₂ is used as a positive activematerial.

Furthermore, the true density of LiCoO₂ is 4.9 to 5.1 g/cm³, whereas thetrue density of LiMn₂O₄ is very low (i.e., 4.0 to 4.2 g/cm³). Therefore,considering the filing property as a positive active material, LiMn₂O₄is more disadvantageous in terms of the capacity.

Furthermore, in a lithium secondary battery using LiMn₂O₄ as a positiveactive material, the structure of LiMn₂O₄ itself is unstable duringcharging/discharging. Therefore, there is a problem that the cyclecharacteristics of the LiMn₂O₄ type battery are worse than those of theLiCoO₂ type battery.

In order to solve the above-mentioned problem, it also is consideredthat a layered lithium-manganese oxide of LiMnO₂ or the like having astructure different from that of LiMn₂O₄ is used as a material for apositive electrode. However, as a result of the detailed study of thisoxide by the inventors of the present invention, it was found that theproperties such as the structure and characteristics are changedremarkably due to the composition of a compound, in particular, thepresence of elements constituting the oxide other than Li and Mn, thekind thereof, and the ratio of quantity thereof, and the process inwhich the oxide is formed.

For example, in the case where the average valence of Mn approaches 3due to the fluctuation of the composition of Spinel typelithium-manganese oxide (LiMn₂O₄), the crystal structure of theabove-mentioned oxide is strained to cause a phase change from theSpinel structure of a cubic to a tetragonal, whereby LiMnO₂ is formed.The phase change from the cubic to the tetragonal occurs along withcharging/discharging in a potential range in the vicinity of 3 V withrespect to lithium. Therefore, the lithium secondary battery using theSpinel type lithium-manganese oxide (LiMn₂O₄) as a material for apositive electrode cannot be used in the same way as in theabove-mentioned lithium secondary battery that is charged/discharged ata voltage in the vicinity of 4 V.

Furthermore, in the case where the structure molar ratio (Li/Mn) is 1,due to the Jahn-Teller effect of trivalent Mn, the crystal structure ofLiMnO₂ exhibits an orthorhombic system.

This compound (LiMnO₂) can be charged/discharged electrochemically at aLi quantity ratio of 0 to 1.0, which results in a discharge capacity ofabout 285 mAh/g in terms of theory. However, as the ratio of tetravalentMn is increased during initial charging, a phase transition to a Spinelstructure occurs. Therefore, the initial charge/discharge curve and thesecond and subsequent charge/discharge curves exhibit different shapes.In addition, the discharge capacity in the case where discharging isterminated at a voltage of 3.5 V or more is decreased remarkably from atheoretical value. Furthermore, the structure is changed with themovement of Mn during charging/discharging. Therefore, cycle durabilityis insufficient, and rapid charging/discharging cannot be conducted.

Therefore, in order to commercialize a layered lithium-manganese oxidesuch as LiMnO₂, it is required to solve the problems involved instabilization of a crystal structure, an increase in capacity due to theenhancement of reversibility of charging/discharging, and durabilityduring a charge/discharge cycle.

DISCLOSURE OF INVENTION

The present invention, in one aspect, provides a lithium-containingcomplex oxide that has a stable structure, is excellent in reversibilityof charging/discharging and durability during a charge/discharge cycle,and has a high energy density per volume, and to provide a non-aqueoussecondary battery using the lithium-containing complex oxide for apositive electrode, which is excellent in durability such as cyclecharacteristics.

More specifically, a lithium-containing complex oxide of the presentinvention includes a composition represented by General Formula:Li_(1+x+α)Ni_((1−x−y+δ/2)Mn_((1−x−y−δ)/2)M_(y)O₂ (where 0≦x≦0.05,−0.05≦x+α≦0.05, 0≦y≦0.4; −0.1≦δ≦0.1 (when 0≦y≦0.2) or −0.24≦δ≦0.24 (when0.2<y≦004); and M is at least one element selected from the groupconsisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge and Sn).

Furthermore, a method for producing a lithium-containing complex oxidehaving a composition represented by General Formula:Li_(1+x+α)Ni_((1−x−y+δ/2)Mn_((1−x−y−δ)/2)M_(y)O₂ (where 0≦x≦0.05,−0.05≦x+α≦0.05, 0≦y≦0.4; −0.1≦δ≦0.1 (when 0≦y≦0.2) or −0.24≦δ≦0.24 (when0.2<y≦0.4); and M is at least one element selected from the groupconsisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge and Sn) is characterized bysintering a complex compound containing at least Ni and Mn asconstituent elements and a Li compound.

Furthermore, a non-aqueous secondary battery of the present inventionincludes a positive electrode made of a positive active material, anegative electrode made of a negative active material, and a non-aqueouselectrolyte, wherein the positive active material is alithium-containing complex oxide having a composition represented byGeneral Formula: Li_(1+x+α)Ni_((1−x−y+δ/2)Mn_((1−x−y−δ)/2)M_(y)O₂ (where0≦x≦0.05, −0.05≦x+α≦0.05, 0≦y≦0.4; −0.1≦δ≦0.1 (when 0≦y≦0.2) or−0.24≦δ≦0.24 (when 0.2<y≦0.4); and M is at least one element selectedfrom the group consisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge and Sn).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing an X-ray diffraction pattern of alithium-containing complex oxide synthesized in Example 1 according tothe present invention.

FIG. 2 is a view showing an X-ray diffraction pattern of alithium-containing complex oxide synthesized in Example 8 according tothe present invention.

FIG. 3 is a view showing an X-ray diffraction pattern of alithium-containing complex oxide synthesized in Example 9 according tothe present invention.

FIG. 4 is a view showing an X-ray diffraction pattern of alithium-containing complex oxide synthesized in Comparative Example 4according to the present invention.

FIG. 5 is a view showing an X-ray diffraction pattern of alithium-containing complex oxide synthesized in Comparative Example 5according to the present invention.

FIG. 6 is a view showing discharge curves of positive electrodes ofbatteries using, for positive electrodes, the lithium-containing complexoxides synthesized in Examples 1, 6, and 8 according to the presentinvention, and Comparative Examples 1 and 2.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail byway of embodiments. The lithium-containing complex oxide of the presentinvention is a complex oxide in a very limited composition range basedon a composition containing at least Ni and Mn as constituent elements,in which the quantity ratio between Ni and Mn is 1:1, represented byGeneral Formula: Li_(1+x+α)Ni_((1−x−y+δ)/2)Mn_((1−x−y−δ)/2)M_(y)O₂(where 0≦x≦0.05, −0.05≦x+α≦0.05, 0≦y≦0.4; −0.1≦δ≦0.1 (when 0≦y≦0.2) or−0.24≦δ≦0.24 (when 0.2≦y≦0.4); and M is at least one element selectedfrom the group consisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge and Sn).

According to the present invention, the reason why a lithium-containingcomplex oxide only in the limited composition range is selected is asfollows. In a lithium-manganese oxide, as described above, when theratio of trivalent Mn is increased, the crystal structure is straineddue to the Jahn-Teller effect, and the potential of charging/dischargingis lowered. Therefore, it is required to set the valence of Mn to beclose to 4. However, as the ratio of tetravalent Mn is increased, aphase transition to a Spinel structure is likely to occur, whichnecessitates stabilization of the crystal structure.

The inventors of the present invention considered that, in order tosolve the above-mentioned problems, it is effective to increase theaverage valence of Mn by allowing an excess amount of Li to be containedin LiMnO₂ or to substitute an element (e.g., Co, Ni, etc.) capable ofstably constituting a layered lithium-containing complex oxide for Mn ofLiMnO₂, and studied the quantity ratio of Li, the kind of substituentelements, and the quantity ratio thereof in detail.

Consequently, the following was found: in a composition range based onthe composition represented by General Formula LiNi_(1/2)Mn_(1/2)O₂ inwhich the quantity ratio between Ni and Mn is ½:½ (i.e., 1:1), Li issubstituted for Ni and Mn by x/2, respectively, the quantity ratiobetween Ni and Mn is shifted from ½:½ by δ/2 and −δ/2, respectively, thequantity ratio of Li has a width of α, and an element M (where M is atleast one element selected from the group consisting of Ti, Cr, Fe, Co,Cu, Zn, Al, Ge and Sn) is substituted for Ni and Ma by y/2,respectively; that is, in a composition range represented by GeneralFormula: Li_(1+x+α)Ni_((1−x−y+δ)/2)Mn_((1−x−y−δ)/2)M_(y)O₂ L (where0≦x≦0.05, −0.05≦x+α≦0.05, 0≦y≦0.4; −0.1≦δ≦0.1 (when 0≦y≦0.2) or−0.24≦δ≦0.24 (when 0.2<y≦0.4); and M is at least one element selectedfrom the group consisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge and Sn), alithium-containing complex oxide whose layered crystal structure isstabilized and which is excellent in reversibility ofcharging/discharging and durability during a charging/discharging cyclein a potential range in the vicinity of 4 V can be obtained. Thefollowing also was found: particularly, in the case of y>0, i.e., in thecase where the element M is added, a lithium-containing complex oxidehaving more excellent characteristics can be obtained.

This is considered to be caused by the following: the average valence ofMn in the lithium-containing complex oxide has a value in the vicinityof 4 (about 3.3 to 4); and movement of Mn in the crystal is suppressedin the course of doping of Li and removal of Li duringcharging/discharging. According to the present invention, as the valenceof Mn, a value measured by an X-ray absorption spectrometer was used.

Furthermore, as described above, when the lithium-containing complexoxide containing at least Ni and Mn as constituent elements, having astable layered structure and being excellent in reversibility ofcharging/discharging and durability during a charging/discharging cycle,is subjected to an X-ray diffraction measurement using a CuKα-ray,diffraction peaks corresponding to those of (003) and (104) of LiNiO₂are present at a diffraction angle 2θ in the vicinity of 18° and 44°,one for each, and in a range of 63° to 66°, two diffraction peakscorresponding to those of (108) and (110) are present. Thus, it wasfound that the above-mentioned lithium-containing complex oxide is asingle-phase complex oxide having the same characteristics as those ofLiNiO₂.

The diffraction pattern was studied in more detail. Consequently, thefollowing also was found: assuming that the areas of the diffractionpeaks in the vicinity of 18° and 44° (i.e., accumulated intensity) areI₁₈ and I₄₄, the ratio I₄₄/I₁₈ is 0.9<I₄₄/I₁₈≦1.2 (when 0≦y≦0.2) or0.7≦I₄₄/I₁₈≦1 (when 0.2<y≦0.4), and the difference θa betweendiffraction angles (2θ) of two diffraction peaks in the above-mentionedrange of 63° to 66° is 0.3°≦θa≦0.6° (when 0≦y≦0.2) or 0.55°≦θa≦0.75°(when 0.2<y≦0.4).

The lithium-containing complex oxide having such a charging/dischargingcurve can be charged/discharged in a voltage range in the vicinity of 4V in the same way as in LiMn₂O₄ having a Spinel structure, and can besubstituted for LiCoO₂ that is a conventional positive active material.

It also was found that the lithium-containing complex oxide having theabove-mentioned composition has a large true density (i.e., 4.55 to 4.95g/cm³) and a high volume energy density. The true density of thelithium-containing complex oxide containing Mn in a predetermined rangeis varied largely depending upon the composition thereof. However, itsstructure is stabilized and a single phase is likely to be formed in theabove-mentioned narrow composition range. Therefore, the above-mentionedlithium-containing complex oxide is considered to have a large truedensity close to that of LiCoO₂. Particularly in the case of acomposition dose to a stoichiometric ratio, the true density becomes alarge value, and in −0.015≦x+α≦0.015, a high-density complex oxide ofabout 4.7 g/cm³ or more is obtained.

Furthermore, as described above, the lithium-containing complex oxide ofthe present invention is based on the composition in which the quantityratio between Ni and Mn is 1:1 as in LiNi_(1/2)Mn_(1/2)O₂. However, thecomposition was studied in more detail, the following was found: alithium-containing complex oxide having particularly excellentcharacteristics can be obtained in the vicinity of the composition wherethe quantity ratio of Ni, Mn and M is 1:1:1 (i.e., the compositionrepresented by General Formula: LiNi_(1/3)Mn_(1/3)M_(1/3)O₂ where y=⅓).

In the above-mentioned General Formula:Li_(1+x+α)Ni_((1−x−y+δ)/2)Mn_((1−x−y−δ)/2)M_(y)O₂ (where M is at leastone element selected from the group consisting of Ti, Cr, Fe, Co, Cu,Zn, Al, Ge and Sn), in the composition range of 0≦y≦0.2, only a smallshift (δ/2) in quantity ratio between Ni and Mn can be permitted,whereas in the composition range of 0.2<y≦0.4, the stability of thecrystal structure becomes higher, and a single phase is likely to beformed. As a result, even if a shift in quantity ratio between Ni and Mnis increased, an intended lithium-containing complex oxide can beobtained. Therefore, in the above-mentioned General Formula, when0≦y≦0.2, the range of δ becomes narrow (i.e., −0.1≦δ≦0.1), whereas when0.2<y≦0.4, the range of δ becomes wide (i.e., −0.24≦δ≦0.24).

Furthermore, in the composition range of 0.2<y≦0.4, the true densitybecomes larger than that of the compound in the composition range of0≦y≦0.2. Therefore, it also was clarified that such a material issuitable for a higher capacity. More specifically, a compound with astoichiometric composition has a true density of about 4.75 to 4.95g/cm³ in the composition range of 0.2<y≦0.4, whereas it has a truedensity of about 4.55 to 4.74 g/cm³ in the composition range of 0≦y≦0.2.

Herein, the reason why the upper limit value of y is set to be 0.4 isthat when the composition at y>0.4, i.e., the substitution amount by theelement M becomes larger than 0.4, a heterogeneous phase is likely to beformed in an intended complex oxide, impairing the stability of thecompound.

The use of a compound in a very wide composition range containing acomposition range of the lithium-containing complex oxide of the presentinvention as a material for a positive electrode of a non-aqueoussecondary battery has already been disclosed in JP 3064655 B, JP 9(1997)-199127 A, JP 10 (1998)-69910 A, JP 2000-294242 A, and the like.However, none of them discloses that a lithium-containing complex oxidehaving particularly excellent characteristics is obtained in a limitedcomposition range where the quantity ratio between Ni and Mn is in thevicinity of 1:1 as disclosed by the present invention. Thus, the presentinvention would not have been obvious over these prior arts.

In the above-mentioned lithium-containing complex oxide, it is verydifficult to obtain a single phase only by mixing a Li compound, a Mncompound, a Ni compound, and the like, followed by sintering.

The reason for this is considered as follows: since the diffusion speedof Ni, Mn and the like in a solid is low, it is difficult to allow themto be diffused uniformly in a synthesis reaction, so that they are notdistributed uniformly in a generated oxide.

The inventors of the present invention also studied a method forsynthesizing the above-mentioned oxide in detail, and consequently,found that a single phase of the lithium-containing complex oxide of thepresent invention can be synthesized relatively easily by sintering acomplex compound containing at least Ni and Mn as constituent elementsand a Li compound. More specifically, a complex compound of constituentelements such as Ni and Mn is synthesized previously, and the resultantcompound is sintered together with a Li compound, whereby the metalelement is distributed uniformly during the reaction of formation of theoxide, and hence, the formation of a single phase can be simplified.Needless to say, the method for synthesizing the lithium-containingcomplex oxide of the present invention is not limited to the abovemethod. However, the physical properties (i.e., the structure stability,reversibility of charging/discharging, true density, and the like) of acomplex oxide to be generated are considered to be varied largelydepending upon which synthesis process is used.

Herein, as the complex compound containing at least Ni and Mn asconstituent elements, for example, a coprecipitated compound containingat least Ni and Mn, a hydrothermally synthesized compound containing atleast Ni and Mn, a mechanically synthesized compound containing at leastNi and Mn, and compounds obtained by heat-treating these compounds maybe used. An oxide or a hydroxide of Ni and Mn such asNi_(0.5)Mn_(0.5)(OH)₂, NiMn₂O₄, and Ni_(0.5)Mn_(0.5)OOH can be usedpreferably. In the case of synthesizing a lithium-containing complexoxide containing M as a constituent element (M is at least one elementselected from the group consisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge andSn), an intended oxide can be obtained by mixing a complex compoundcontaining at least Ni and Mn, a Li compound, and a compound containingM, followed by sintering. If possible, it is preferable to use a complexoxide containing M in addition to Ni and Mn from the beginning. Thequantity ratio of Ni, Mn and M in the above-mentioned complex compoundshould be selected appropriately in accordance with the composition ofan intended lithium-containing complex oxide.

As the Li compound, various lithium salts can be used. Examples of thelithium salts include lithium hydroxide.monohydrate, lithium nitrate,lithium carbonate, lithium acetate, lithium bromide, lithium chloride,lithium citrate, lithium fluoride, lithium iodide, lithium lactate,lithium oxalate, lithium phosphate, lithium pyruvate, lithium sulfate,lithium oxide, and the like. Among them, lithium hydroxide.monohydrateis used most preferably because it does not generate gas that adverselyaffects an environment, such as carbon dioxide, nitrogen oxide, andsulfur oxide.

The complex compound containing at least Ni and Mn as constituentelements and the Li compound are mixed in a ratio substantially inaccordance with the composition of an intended lithium-containingcomplex oxide. For example, the mixture is sintered in an atmospherecontaining oxygen at 700° C. to 1100° C. for 1 to 24 hours, whereby alithium-containing complex oxide of the present invention can besynthesized.

Regarding the heat treatment for the above-mentioned sintering, it ispreferable that the mixture is once heated to a temperature (about 250°C. to 850° C.) lower than a sintering temperature instead of beingheated to a predetermined temperature directly, pre-heated at thetemperature, and further heated to a sintering temperature so as toeffect a reaction. The reason for this is as follows: in the generationprocess of the lithium-containing complex oxide of the presentinvention, the reaction between the Li compound and the complex compoundcontaining at least Ni and Mn as constituent elements is effected instages, and the lithium-containing complex oxide is generated finallyvia an intermediate product. More specifically, in the case where themixture is heated to a sintering temperature directly, the Li compoundand the complex compound containing at least Ni and Mn as constituentelements are reacted partially up to the final stage, and thelithium-containing complex oxide thus generated may hinder the reactionof an unreacted substance and impair the uniformity of the composition.Furthermore, in order to shorten a time required for a reaction processand obtain a homogeneous lithium-containing complex oxide, it iseffective to conduct heating in stages. The time for pre-heating is notparticularly limited; however, it should be conducted generally forabout 0.5 to 30 hours.

Furthermore, in the process of sintering the mixture of the Li compoundand the complex compound containing at least Ni and Mn as constituentelements, a mixture obtained in a dry state may be used as it is.However, it is preferable that the mixture is dispersed in a solventsuch as ethanol to form a slurry, and mixed in a planet type ball millfor about 30 to 60 minutes, followed by drying. This is because thehomogeneity of the lithium-containing complex oxide to be synthesized isenhanced further.

The above-mentioned heat treatment should be conducted in an atmospherecontaining oxygen, i.e., in the air, in an environment of a mixture ofinert gas (e.g., argon, helium, nitrogen, etc.) and oxygen gas, or inoxygen gas. The ratio of the oxygen in the environment preferably is setat 10% or more by volume.

The flow rate of the above-mentioned gas is preferably 1 dm³/min. per100 g of the mixture, more preferably 1 to 5 dm³/min. In the case wherethe gas flow rate is small, i.e., in the case where the gas flow speedis low, a reaction is effected non-uniformly, whereby an impurity suchas Mn₂O₃ and Li₂MnO₃ is likely to be generated.

By using the lithium-containing complex oxide of the present inventionobtained by the above-mentioned method as a positive active material, anon-aqueous secondary battery is produced, for example, as follows.

As a positive electrode, a positive electrode mixture is used as it is,which is obtained by adding, if required, a conductive assistant such asscaly graphite and acetylene black and a binder such aspolytetrafluoroethylene and vinylidene polyfluoride to theabove-mentioned lithium-containing complex oxide. Alternatively, asubstrate that also functions as a charge collector is coated with orimpregnated with the positive electrode mixture, whereby the positiveelectrode mixture is used under the condition of being integrated withthe substrate. Examples of the substrate include a net of metal such asaluminum, stainless steel, titanium and copper; punching metal, expandedmetal, foamed metal, metal foil, and the like.

As a positive active material, the above-mentioned lithium-containingcomplex oxide may be used by itself. In addition, the above-mentionedlithium-containing complex oxide may be mixed with another activematerial, or the lithium-containing complex oxide may be used as acomplex with another active material. For example, the electronconductivity of the above-mentioned lithium-containing complex oxide isinferior to that of the lithium-containing cobalt oxide such as LiCoO₂.Therefore, a voltage drop is likely to be increased during dischargingof a large current or at the end of discharging. However, by using alithium-containing cobalt oxide excellent in electron conductivity mixedwith the lithium-containing complex oxide, the voltage drop can besuppressed, whereby discharge characteristics can be enhanced. As thelithium-containing cobalt oxide, not only LiCoO₂ but also a compoundsuch as LiCo_(1−t)Ni_(t)O₂ obtained by substituting another element(e.g., Ni) for a part of Co of LiCoO₂ can be used. In this case, if theratio of the lithium-containing cobalt oxide is increased too much,durability such as high-temperature storage characteristics is likely tobe decreased. Therefore, it is required that the ratio of thelithium-containing cobalt oxide is set to be 50% or less by mass, basedon the entire active material.

Furthermore, as a negative active material to be opposed to the positiveelectrode, lithium or a lithium-containing compound generally is used.Examples of the lithium-containing compound include a lithium alloy suchas a Li—Al alloy, a Li—Pb alloy, a Li—In alloy and a Li—Ga alloy;elements capable of forming an alloy with lithium such as Si, Sn, and aMg—Si alloy; and an alloy mainly containing these elements. Furthermore,a carbon material such as graphite and fibrous carbon, alithium-containing complex nitride, and the like can be used in additionto an oxide material such as a Sn oxide and a Si oxide. Furthermore, theabove-mentioned plurality of materials may be combined, and a complex ofa carbon material and Si also is used preferably. The same method asthat of the positive electrode also applies to the production of thenegative electrode.

Although varied depending upon the kind of a negative active material,the ratio between the positive and negative active materials generallyis set as (mass of a positive active material)/(mass of a negativeactive material)=1.5 to 3.5, whereby the characteristics of thelithium-containing complex oxide can be utilized. In the case of using,as a negative active material, elements capable of forming an alloy withlithium, an alloy mainly containing these elements, a lithium-containingcomplex nitride, or a complex of these materials with another componentsuch as a carbon material, the capacity of the negative electrodebecomes too large at the above-mentioned ratio. Therefore, it isdesirable to set (mass of a positive active material)/(mass of anegative active material) to be 4 to 7.

Examples of a non-aqueous electrolyte in the non-aqueous secondarybattery of the present invention include an organic solvent type liquidelectrolyte in which an electrolyte is dissolved in an organic solvent(i.e., an electrolytic solution), a polymer electrolyte in which theelectrolytic solution is held in a polymer, and the like. Although thereis no particular limit to an organic solvent contained in theelectrolytic solution or the polymer electrolyte, it is preferable thatthe organic solvent contains a chain ester in terms of the loadcharacteristics. Examples of the chain ester include chain carbonatesuch as dimethyl carbonate, diethyl carbonate and ethyl methylcarbonate; ethyl acetate; and methyl propionate. These chain esters maybe used alone or in combination. Particularly, in order to enhancelow-temperature characteristics, it is preferable that theabove-mentioned chain ester occupies 50% by volume of the entire organicsolvent. It is particularly preferable that the chain ester occupies 65%by volume or more of the entire organic solvent.

The organic solvent preferably is composed of the above-mentioned chainester mixed with another ester having a high dielectric constant (30 ormore), so as to enhance a discharge capacity, instead of being composedof only the above-mentioned chain ester. Examples of such an esterinclude cyclic carbonate such as ethylene carbonate, propylenecarbonate, butylene carbonate and vinylene carbonate, γ-butylolactone,ethylene glycol sulfite, and the like. In particular, an ester with acyclic structure such as ethylene carbonate and propylene carbonate ismore preferable.

An ester with such a high dielectric constant is contained preferably inan amount of 10% by volume, more preferably 20% by volume, based on theentire organic solvent, in terms of the discharge capacity. Furthermore,in terms of the load characteristics, the ester is contained preferablyin an amount of 40% by volume or less, more preferably 30% by volume orless.

Furthermore, examples of the solvent that can be used other than theester having a high dielectric constant include 1,2-dimethoxyethane,1,3-dioxolane, tetrahydrofuran, 2-methyl-tetrahydrofuran, diethylether,and the like. In addition, an amineimide type organic solvent, asulfur-containing or fluorine-containing organic solvent, and the likecan be used.

As an electrolyte to be dissolved in the organic solvent, for example,LiClO₄, LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, LiCF₃SO₃, LiC₄F₉SO₃, LiCF₃CO₂,Li₂C₂F₄(SO₃)₂, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, LiC_(n)F_(2n+1)SO₃ (n≧2), andthe like are used alone or in combination. Among them, LiPF₆, LiC₄F₉SO₃,and the like that allow satisfactory charge/discharge characteristics tobe obtained are used preferably. Although there is no particular limitto the concentration of the electrolyte in the electrolytic solution,the concentration is preferably about 0.3 to 1.7 mol/dm³, morepreferably about 0.4 to 1.5 mol/dm³.

Furthermore, for the purpose of enhancing the safety and storagecharacteristics of a battery, an aromatic compound may be contained in anon-aqueous electrolytic solution. As the aromatic compound, benzeneshaving an alkyl group such as cydohexylbenzene and t-butylbenzene,biphenyl, or fluorobenzenes are used preferably

It is preferable that a separator has sufficient strength and is capableof holding a large quantity of electrolytic solution. In view of this, amicro-porous film made of polypropylene, polyethylene, polyolefin suchas a copolymer of propylene and ethylene, a nonwoven fabric, and thelike with a thickness of 5 to 50 μm are used preferably. Particularly,in the case of using a thin separator (5 to 20 μm), batterycharacteristics such as a charge/discharge cycle and high-temperaturestorage are likely to be degraded. However, since the lithium-containingcomplex oxide of the present invention is excellent in safety, even ifsuch a thin separator is used, a battery is allowed to function withstability.

Hereinafter, the present invention will be described by way of examples.The present invention is not limited to the examples.

EXAMPLE 1

Ammonium water with its pH adjusted to about 12 by addition of sodiumhydroxide was placed in a reaction container. While strongly stirringthe ammonium water, a mixed aqueous solution containing nickel sulfateand manganese nitrate in an amount of 1 mol/dm³, respectively, and 25%by mass of ammonium water were dropped onto the reaction container at 46cm³/min. and 3.3 cm³/min., respectively, by using a metering pump,whereby a coprecipitated compound of Ni and Mn was generated. At thistime, a sodium hydroxide aqueous solution with a concentration of 3.2mol/dm³ also was dropped simultaneously so that the temperature of thereaction solution was kept at 50° C. and the pH thereof was kept atabout 12. Furthermore, the reaction was effected while nitrogen gas waspurged at a rate of 1 dm³/min. so that the atmosphere of the reactionsolution became inactive.

The product thus obtained was washed with water, filtered and dried toobtain a hydride containing Ni and Mn in a ratio of 1:1. Then, 0.2 molof the hydride and 0.198 mol of LiOH.H₂O were obtained, and the mixturewas dispersed in ethanol to form a slurry. Thereafter, the slurry wasmixed in a planet type ball mil for 40 minutes, and dried at roomtemperature to prepare a mixture. Then, the mixture was placed in acrucible made of alumina, and heated to 800° C. at an air flow of 1dm³/min. The mixture was kept at that temperature for 2 hours, wherebypreheating was conducted. The temperature was raised further to 1000°C., and the mixture was sintered for 12 hours, whereby alithium-containing complex oxide was synthesized. The prepared compoundwas ground in a mortar and stored as powder in a desicator.

The above-mentioned oxide powder was measured for a composition by anatomic absorption spectrometer, revealing that the composition wasrepresented by Li_(0.99)Ni_(0.5)Mn_(0.5)O₂. Furthermore, in order toanalyze the state of the above-mentioned compound, X-ray absorptionspectroscopy WXAS) of Mn was conducted, using BL4 beam port ofsuperconducting small radiation source “Aurora” (produced by SumitomoElectric Industries, Ltd.) at SR center of Ritsumeikan University. Thedata thus obtained was analyzed with analysis software “REX” (producedby Rigaku Denki), based on the Literature (Journal of theElectrochemical Society, 146 P2799-2809 (1999)). In order to determinethe valence of Mn of the above-mentioned compound, as reference samples,MnO₂ and LiNi_(0.5)Mn_(1.5)O₄ (both of them are reference samples ascompounds having Mn of which average valence is 4), LiMn₂O₄ (referencesample as a compound having Mn of which average valence is 3.5), LiMnO₂and Mn₂O₃ (both of them are reference samples as compounds having Mn ofwhich average valence is 3), and MnO (reference sample as a compoundhaving Mn of which average valence is 2) were used. A regression linerepresenting the relationship between the K absorption edge position ofn of each reference sample and the valence of Mn was obtained. As aresult, the K absorption edge position of MnO₂ was substantially thesame as that of LiNi_(0.5)Mn_(1.5)O₄. Therefore, the average valence ofMn of the above-mentioned compound was determined to be about 4.

Regarding Ni, it was not possible to obtain a compound appropriate as areference sample having Ni with a valence of 3 or more, so that itsexact valence was not obtained. However, the K absorption edge positionof the reference sample having Ni was substantially the same as that ofNiO and LiNi_(0.5)Mn_(1.5)O₄ that are compounds having Ni of whichaverage valence is 2. Therefore, it was assumed that the average valenceof Ni of the above-mentioned compound is about 2.

EXAMPLE 2

First, 0.198 mol of hydroxide containing Ni and n in a ratio of 1:1,synthesized in the same way as in Example 1 and 0.202 mol of LiOH.H₂Owere obtained, and a lithium-containing complex oxide represented byLi_(1.01)Ni_(0.495)Mn_(0.495)O₂ was synthesized in the same way as inExample 1.

EXAMPLE 3

First, 0.196 mol of hydroxide containing Ni and Mn in a ratio of 1:1,synthesized in the same way as in Example 1 and 0.204 mol of LiOH.H₂Owere obtained, and a lithium-containing complex oxide represented byLi_(1.02)Ni_(0.49)Mn_(0.49)O₂ was synthesized in the same way as inExample 1.

EXAMPLE 4

First, 0.194 mol of hydroxide containing Ni and Mn in a ratio of 1:1,synthesized in the same way as in Example 1 and 0.206 mol of LiOH.H₂Owere obtained, and a lithium-containing complex oxide represented byLi_(1.03)Ni_(0.485)Mn_(0.485)O₂ was synthesized in the same way as inExample 1.

EXAMPLE 5

First, 0.192 mol of hydroxide containing Ni and Mn in a ratio of 1:1,synthesized in the same way as in Example 1 and 0.208 mol of LiOH. H₂Owere obtained, and a lithium-containing complex oxide represented byLi_(1.04)Ni_(0.48)Mn_(0.48)O₂ was synthesized in the same way as inExample 1.

EXAMPLE 6

First, 0.19 mol of hydroxide containing Ni and Mn in a ratio of 1:1,synthesized in the same way as in Example 1 and 0.21 mol of LiOH.H₂Owere obtained, and a lithium-containing complex oxide represented byLi_(1.05)Ni_(0.475)Mn_(0.475)O₂ was synthesized in the same way as inExample 1.

EXAMPLE 7

A hydroxide containing Ni, Mn and Co in a ratio of 4.5:4.5:1 wassynthesized in the same way as in Example 1, except that a mixed aqueoussolution containing nickel sulfate, manganese nitrate and cobalt sulfatein a ratio of 0.9 mol/dm³, 0.9 mol/dm³ and 0.2 mol/dm³, respectively,was dropped. A lithium-containing complex oxide represented byLi_(0.99)Ni_(0.45)Mn_(0.45)Co_(0.1)O₂ was synthesized in the same way asin Example 1.

EXAMPLE 8

A lithium-containing complex oxide represented byLi_(0.99)Ni_(0.375)Mn_(0.375)Co_(0.25)O₂ was synthesized in the same wayas in Example 1, except that a mixed aqueous solution containing nickelsulfate, manganese nitrate and cobalt sulfate in a ratio of 0.75mol/dm³, 0.75 mol/dm⁸ and 0.5 mol/dm³, respectively, was dropped.

EXAMPLE 9

A lithium-containing complex oxide represented byLi_(0.99)Ni_(0.34)Mn_(0.33)Co_(0.33)O₂ was synthesized in the same wayas in Example 1, except that a mixed aqueous solution containing nickelsulfate, manganese nitrate and cobalt sulfate in a ratio of 0.67mol/dm³, 0.66 mol/dm³ and 0.66 mol/dm³, respectively, was dropped.

EXAMPLE 10

A lithium-containing complex oxide represented byLi_(0.99)Ni_(0.3)Mn_(0.3)Co_(0.4)O₂ was synthesized in the same way asin Example 1, except that a mixed aqueous solution containing nickelsulfate, manganese nitrate and cobalt sulfate in a ratio of 0.6 mol/dm³,0.6 mol/dm³ and 0.8 mol/dm³, respectively, was dropped.

COMPARATIVE EXAMPLE 1

First, 0.2 mol of LiOH.H₂O and 0.2 mol of MnOOH were obtained and mixedin a planet type ball mil for 30 min. to obtain a mixture. The mixturewas placed in a crucible made of alumina. The mixture was sintered at450° C. for 10 hours at a nitrogen stream of 1 dm³/min, wherebyorthorhombic lithium-manganese oxide represented by LiMnO₀ wassynthesized.

COMPARATIVE EXAMPLE 2

0.18 mol of hydroxide containing Ni and Mn in a ratio of 1:1 synthesizedin the same way as in Example 1 and 0.22 mol of LiOH.H₂O were obtained,and a lithium-containing complex oxide represented byLi_(1.1)Ni_(0.45)Mn_(0.45)O₂ was synthesized in the same way as inExample 1.

COMPARATIVE EXAMPLE 3

A lithium-containing complex oxide represented byLi_(0.99)Ni_(0.25)Mn_(0.25)Co_(0.5)O₂ was synthesized in the same way asin Example 1, except that a mixed aqueous solution containing nickelsulfate, manganese nitrate and cobalt sulfate in an amount of 0.5mol/dm³, 0.5 mol/dm³ and 1 mol/dm³, respectively, was dropped.

COMPARATIVE EXAMPLE 4

A lithium-containing complex oxide represented byLi_(0.99)Ni_(0.2)Mn_(0.2)Co_(0.6)O₂ was synthesized in the same way asin Example 1, except that a mixed aqueous solution containing nickelsulfate, manganese nitrate and cobalt sulfate in an amount of 0.4mol/dm³, 0.4 mol/dm³ and 1.2 mol/dm³, respectively, was dropped.

COMPARATIVE EXAMPLE 5

A lithium-containing complex oxide represented byLi_(0.99)Ni_(0.25)Mn_(0.75)O₂ was synthesized in the same way as inExample 1, except that a mixed aqueous solution containing nickelsulfate and manganese nitrate in an amount of 0.5 mol/dm³ and 1.5mol/dm³, respectively, was dropped.

COMPARATIVE EXAMPLE 6

A lithium-containing complex oxide represented byLi_(0.99)Ni_(0.6)Mn_(0.3)Co_(0.1)O₂ was synthesized in the same way asin Example 7, except that the ratio between nickel sulfate and manganesenitrate is 1.2 mol/dm³ and 0.6 mol/dm³, respectively. More specifically,the lithium-containing complex oxide in Comparative Example 6 isdifferent from Example 7 only in an amount ratio between Ni and Mn.

REFERENCE EXAMPLE

First, 0.2 mol of LiOH.H₂O and 0.1 mol of Ni(OH)₂, and 0.1 mol of MnOOHwere obtained and mixed in a planet type ball mill for 30 minutes toobtain a mixture. The mixture was placed in a crucible made of alumina,and sintered in the air at 800° C. for 10 hours, whereby alithium-containing complex oxide represented by LiNi_(0.5)Mn_(0.5)O₂ wassynthesized.

Table 1 shows the list of the respective lithium-containing complexoxides synthesized in Examples 1 to 10, Comparative Examples 1 to 6, andReference Example.

TABLE 1 Composition [Li_(1+x+α)Ni_((1−x−y+δ)/2)Mn_((1−x−y−δ)/2)M_(y)O₂]x x + α y δ Example 1 Li_(0.99)Ni_(0.5)Mn_(0.5)O₂ 0 −0.01 0 0 Example 2Li_(1.01)Ni_(0.495)Mn_(0.495)O₂ 0.01 0.01 0 0 Example 3Li_(1.02)Ni_(0.49)Mn_(0.49)O₂ 0.02 0.02 0 0 Example 4Li_(1.03)Ni_(0.485)Mn_(0.485)O₂ 0.03 0.03 0 0 Example 5Li_(1.04)Ni_(0.48)Mn_(0.48)O₂ 0.04 0.04 0 0 Example 6Li_(1.05)Ni_(0.475)Mn_(0.475)O₂ 0.05 0.05 0 0 Example 7Li_(0.99)Ni_(0.45)Mn_(0.45)Co_(0.1)O₂ 0 −0.01 0.1 0 Example 8Li_(0.99)Ni_(0.375)Mn_(0.375)Co_(0.25)O₂ 0 −0.01 0.25 0 Example 9Li_(0.99)Ni_(0.34)Mn_(0.33)Co_(0.33)O₂ 0 −0.01 0.33 0.01 Example 10Li_(0.99)Ni_(0.3)Mn_(0.3)Co_(0.4)O₂ 0 −0.01 0.4 0 Comparative LiMnO₂ 0 00 −1 Example 1 Comparative Li_(1.1)Ni_(0.45)Mn_(0.45)O₂ 0.1 0.1 0 0Example 2 Comparative Li_(0.99)Ni_(0.25)Mn_(0.25)Co_(0.5)O₂ 0 −0.01 0.50 Example 3 Comparative Li_(0.99)Ni_(0.2)Mn_(0.2)Co_(0.6)O₂ 0 −0.01 0.60 Example 4 Comparative Li_(0.99)Ni_(0.25)Mn_(0.75)O₂ 0 −0.01 0 −0.5Example 5 Comparative Li_(0.99)Ni_(0.6)Mn_(0.3)Co_(0.1)O₂ 0 −0.01 0.10.3 Example 6 Reference LiNi_(0.5)Mn_(0.5)O₂ 0 0 0 0 Example

The above-mentioned lithium-containing complex oxides in Examples 1 to10 of the present invention, Comparative Examples 1 to 6, and ReferenceExample were subjected to X-ray diffraction measurement using a CuKαline. The lithium-containing complex oxides in Examples 1 to 10 of thepresent invention, Comparative Examples 2 to 6, and Reference Exampleexhibited X-ray diffraction patterns similar to that of LiNiO₂ having alayered structure, whereas a peak representing the generation of aheterogeneous phase was recognized in the X-ray diffraction patterns inComparative Examples 3 to 5 and Reference Example. Furthermore, theX-ray diffraction pattern in Comparative Example 1 is an orthorhombicpattern different from that of LiNiO₂. In Examples 1 to 10 of thepresent invention, and Comparative Examples 2 and 6, a peak caused bythe generation of a heterogeneous phase was not recognized. Morespecifically, there was one diffraction peak where a diffraction angle2θ was in the vicinity of 18°, there was one diffraction peak where adiffraction angle 2θ was in the vicinity of 44°, and there were twodiffraction peaks where the diffraction angle 2θ was present in thevicinity of 63° to 66°. Thus, the obtained oxide was recognized to be asingle phase of the lithium-containing complex oxide having a structuresimilar to that of LiNiO₂. Ea the diffraction peak where the diffractionangle 2θ was present in a range of 63° to 66°, a peak by a Kα₂ line alsowas recognized adjacent to a peak by a Kα₁ line of Cu. According to thepresent invention, as the diffraction peak where the diffraction angle2θ was present in a range of 63° to 66°, only a peak by the Kα₁ line isconsidered.

Among the above diffraction patterns, the X-ray diffraction patterns inExamples 1, 8, 9, Comparative Example 4 and 5 are illustrated in FIGS. 1to 5.

Table 2 shows values obtained by measuring the ratio (I₄₄/I₁₈) betweenthe accumulated intensities I₁₈ and I₄₄ of the diffraction peaks in thevicinity of 18° and 44°, and the difference θa in diffraction anglebetween two diffraction peaks in a range of 63° to 66°. Thelithium-containing complex oxide in Comparative Example 1 is differentfrom that of the present invention in crystal structure. In thelithium-containing complex oxides in Comparative Examples 3 to 5 andReference Example, at least three diffraction peaks were present in arange of 63° to 66° due to the generation of a heterogeneous phase.Therefore, Table 2 does not describe the data of these compounds.

TABLE 2 X-ray diffraction measurement Ratio of accumulated intensityDifference in (I₄₄/I₁₈) diffraction angle 2θ (°) Example 1 1.13 0.313Example 2 1.14 0.348 Example 3 1.10 0.390 Example 4 1.11 0.435 Example 51.08 0.510 Example 6 1.06 0.555 Example 7 1.04 0.553 Example 8 0.880.672 Example 9 0.83 0.700 Example 10 0.77 0.617 Comparative 0.99 0.625Example 2 Comparative 0.83 0.600 Example 6

In the lithium-containing complex oxides in Examples 1 to 7 where0≦y≦0.2, the accumulated intensity ratio I₄₄/I₁₈ was in a range of 0.9to 1.2, and the diffraction angle difference θa was in a range of 0.3°to 0.6°. Furthermore, in Examples 8 to 10 where 0.2<y≦0.4, I₄₄/I₁₈ wasin a range of 0.7 to 1, and θa was in a range of 0.55° to 0.75°. On theother hand, in Comparative Examples 2 and 6 where the composition wasout of the range of the present invention, I₄₄/I₁₈ or θa deviated fromthe above range. In Comparative Examples 3 to 5 and Reference Example,at least three diffraction peaks were present in a range of 63° to 66°.

Next, the lithium-containing complex oxides in Examples 1 to 10 of thepresent invention, Comparative Examples 1 to 6, and Reference Examplewere measured for the true density, using a true density measurementapparatus. Table 3 shows the results. The measurement error was −0.03g/cm³ at maximum.

TABLE 3 True density (g/cm³) Example 1 4.74 Example 2 4.72 Example 34.68 Example 4 4.65 Example 5 4.62 Example 6 4.57 Example 7 4.75 Example8 4.76 Example 9 4.80 Example 10 4.82 Comparative Example 1 4.20Comparative Example 2 4.38 Comparative Example 3 4.83 ComparativeExample 4 4.90 Comparative Example 5 4.46 Comparative Example 6 4.65Reference Example 4.61

In the lithium-containing complex oxides in Examples 1 to 10 of thepresent invention, the true density was 4.57 to 4.82 g/cm³.Particularly, in Examples 1, 2, and 7 to 10 where the oxide hadsubstantially a stoichiometric composition (i.e., −0.015≦x+α≦0.015), thetrue density had a large value (i.e., 4.7 g/cm³ or more). Among them, inExamples 8 to 10 where a substitution amount y of the element M was0.2<y≦0.4, the largest true density (i.e., 4.76 g/cm³ or more) wasobtained.

On the other hand, in Comparative Example 1 using a conventionalorthorhombic complex oxide, and Comparative Example 2 where the oxidehad a composition remarkably shifted from the stoichiometriccomposition, the true density had a small value (i.e., 4.5 g/cm³ orless). In Comparative Examples 5 and 6 where the ratio between Ni and Mnwas out of the range of the present invention, irrespective of whetherthe oxide had substantially a stoichiometric composition, the truedensity was decreased, compared with Examples 1, 2, and 7 to 10 of thepresent invention. Furthermore, the lithium-containing complex oxide inReference Example had poor homogeneity due to the generation of aheterogeneous phase or the remaining unreacted substance. Therefore, thetrue density in Reference Example was decreased, compared with thelithium-containing complex oxide in Example 1.

Herein, the true densities of the lithium-containing complex oxides inComparative examples 3 and 4 were higher than those of the examples ofthe present invention. This is not because complex oxides with the truedensity shown in Table 3 were obtained as a single phase, but becauseabout 5.1 g/cm³ of LiCoO₂ was generated as a heterogeneous phase.

Next, the lithium-containing complex oxides in Examples 1 to 10 of thepresent invention and Comparative Examples 1 and 2 were measured fordischarge capacity by the following method.

First, 250 parts by mass of N-methyl-2-pyrrolidone were added to 20parts by mass of polyvinylidene fluoride as a binder, and the mixturewas heated to 60° C. to dissolve polyvinylidene fluoride inN-methyl-2-pyrrolidone, whereby a binder solution was prepared. Theabove-mentioned lithium-containing complex oxide was added in an amountof 450 parts by mass as a positive active material to the bindersolution. Furthermore, 5 parts by mass of carbon black and 25 parts bymass of graphite were added, as a conductive assistant, to the resultantmixture, followed by stirring to prepare a coating in the form of aslurry. The coating thus obtained was applied to both surfaces of analuminum foil (thickness: 20 μm) uniformly, and dried. Thereafter, thealuminum foil with the coating applied thereto was subjected to pressureforming by a roller press. The resultant aluminum foil was cut to aband-shaped positive electrode (483 mm×54 mm) having an averagethickness of 190 μm.

A separator made of a microporous polyethylene film (thickness: 25 μm)was placed between the positive electrode produced as described aboveand a negative electrode made of a lithium foil. A non-aqueous solutionwas used as an electrolyte, in which LiPF₆ was dissolved in aconcentration of 1.0 mol/dm³ in a mixed solvent containing ethylenecarbonate and ethyl methyl carbonate in a volume ratio of 1:3. Areference electrode of lithium was placed. Thus, a battery forevaluating the discharge capacity of the positive electrode wasassembled.

The above battery was charged to 4.3 V at a current density withrespective to the area of the positive electrode of 0.2 mA/cm², anddischarged to 3.1 V at the same current density, whereby the dischargecapacity was measured. Table 4 shows values obtained by converting themeasured discharge capacities into values per unit mass (mAh/g) and perunit volume (mAh/cm³) of the positive active material. FIG. 6 showsdischarge curves of the positive electrodes of the batteries using thelithium-containing complex oxides in Examples 1, 6, 8, and ComparativeExamples 1 and 2.

TABLE 4 Discharge capacity Positive active material (mAh/g) (mAh/cm³)Example 1 148 702 Example 2 145 684 Example 3 143 669 Example 4 141 656Example 5 139 642 Example 6 136 622 Example 7 150 713 Example 8 152 724Example 9 153 734 Example 10 153 737 Comparative Example 1 70 294Comparative Example 2 112 491

The lithium-containing complex oxides in Examples 1 to 10 of the presentinvention are capable of being operated at a high discharge potential of3.5 V or more, and exhibited a large discharge capacity (i.e., 136 to153 mAh/g). In contrast, the lithium-containing complex oxides inComparative Examples 1 and 2 had a discharge capacity of 130 mAh/g orless, and had a true density smaller than those of the presentinvention. Therefore, when converted to a discharge capacity per unitvolume, the difference in discharge capacity between the oxides of thepresent invention and those in Comparative Examples 1 and 2 became moreconspicuous.

Furthermore, in order to evaluate the characteristics of theabove-mentioned lithium-containing complex oxides as a non-aqueoussecondary battery, a non-aqueous secondary battery was produced in thefollowing structure.

EXAMPLE 11

By using the lithium-containing complex oxides in Examples 1 and 9independently as a positive active material, non-aqueous secondarybatteries were produced. A positive electrode was obtained by coating analuminum foil substrate with a paste produced by mixing 92 parts by massof the positive active material, 4.5 parts by mass of artificialgraphite, 0.5 parts by mass of carbon black, and 3 parts by mass ofpolyvinylidene fluoride, followed by drying and pressure-forming.

A negative electrode was obtained by coating a copper foil substratewith a paste produced by mixing 92 parts by mass of artificial graphite,3 parts by mass of low-crystalline carbon, and 5 parts by mass ofpolyvinylidene fluoride.

The above-mentioned positive electrode and negative electrode were woundvia a separator made of a microporous polyethylene film (thickness: 16μm). As an electrolyte, a mixed solvent containing ethylene carbonateand ethyl methyl carbonate (volume ratio: 1:2) with LiPF₆ dissolved in aconcentration of 1.2 mol/dm³ was used. Thus, a cylindrical non-aqueoussecondary battery with a capacity of 600 mAh was produced. The massratio between the positive active material and the negative activematerial [(mass of the positive active material)/(mass of the negativeactive material)] was set at 1.9.

EXAMPLE 12

A non-aqueous secondary battery was produced in the same way as inExample 11, except that 70% by mass of the lithium-containing complexoxide in Example 1 and 30% by mass of LiCoO₂ were mixed as a positiveactive material.

COMPARATIVE EXAMPLE 7

A non-aqueous secondary battery was produced in the same way as inExample 11, except that the lithium-containing complex oxide inComparative Example 6, LiCoO₂ and LiNi_(0.8)Co_(0.2)O₂ used for acommercially available non-aqueous secondary battery were usedindependently as a positive active material.

The non-aqueous secondary batteries in Examples 11 and 12, andComparative Example 7 were evaluated for cycle characteristics andhigh-temperature storage characteristics. The cycle characteristics wereevaluated based on the ratio (capacity retention ratio (%)) of adischarge capacity after 100 cycles with respect to a discharge capacityin an initial stage of the cycle when charging/discharging was conductedat a current value of 1C (600 mA). The high-temperature storagecharacteristics were evaluated based on the change in discharge capacitybefore and after storage when the batteries were retained at 60° C. for20 days. More specifically, the high-temperature storage characteristicswere evaluated based on the ratio (capacity retention ratio (%)) of adischarge capacity after storage with respect to a discharge capacitybefore storage, obtained by comparing the discharge capacities whencharging/discharging was conducted at a current value of 1C before andafter storage. Table 5 shows the results of the evaluation of thesecharacteristics.

TABLE 5 Capacity retention ratio (%) Positive active CycleHigh-temperature Battery material characteristics storagecharacteristics Example 11 Example 1 96 97 Example 9 98 98 Example 12Example 1 + 94 96 LiCoO₂ Comparative Comparative 92 87 Example 7 Example6 LiCoO₂ 88 91 LiNi_(0.8)Co_(0.2)O₂ 93 90

The non-aqueous secondary batteries in Examples 11 and 12 using thelithium complex oxide of the present invention as a positive activematerial exhibited excellent cycle characteristics and high-temperaturestorage characteristics, irrespective of using a thin separator(thickness: 16 μm). On the other hand, the non-aqueous secondary batteryin Comparative Example 6 in which the composition was out of the rangeof the present invention and the non-aqueous secondary battery inComparative Example 7 using LiCoO₂ or LiNi_(0.8)Co_(0.2)O₂ used in acommercially available non-aqueous secondary battery as a positiveactive material exhibited cycle characteristics and high-temperaturestorage characteristics that were less excellent than those of thepresent invention.

Furthermore, the batteries in Examples 11 and 12 were discharged at acurrent value of 2C (1200 mA) so as to check the characteristics thereofdung discharge at a large current. As a result, the discharge capacityof the battery in Example 11 was 525 mAh, whereas that of the battery inExample 12 was 573 mAh. Thus, remarkable enhancement of thecharacteristics was recognized. This is because a lithium-containingcobalt oxide was mixed in the lithium-containing complex oxide of thepresent invention.

EXAMPLE 13

Furthermore, a non-aqueous secondary battery was produced by using acomplex of Si and a carbon material as a negative active material. Sipowder and artificial graphite were mixed in a planet type ball mill toform a complex, and the resultant complex was sifted to obtain anegative active material. As a positive active material, thelithium-containing complex oxide in Example 1 was used. Thus, anon-aqueous secondary battery was produced in the same way as in Example11, except that the above-mentioned active materials were used. The massratio between the positive active material and the negative activematerial was set at 6.6. In this battery, the mass ratio of the positiveactive material was increased due to the use of a high-capacity materialas the negative active material. Therefore, the discharge capacity wasincreased by about 7% with the same size as that in Example 11.

The above-mentioned non-aqueous secondary battery was measured for adischarge capacity at a discharge current of 2C to obtain 605 mAh. Thus,the battery had excellent characteristics even during discharge at alarge current. This is because the load on the positive active materialduring discharge was reduced due to the increased mass ratio of thepositive active material, resulting in a decrease in a voltage drop.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, because of thecomposition represented by General Formula:Li_(1+x+α)Ni_((1−x−y+δ)/2)Mn_((1−x−y−δ)/2)M_(y)O₂ (where 0≦x≦0.05,−0.05≦x+α≦0.05, 0≦y≦0.4; −0.1≦δ≦0.1 (when 0≦y≦0.2) or −0.24≦δ≦0.24 (when0.2<y≦0.4); and M is at least one element selected from the groupconsisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge and Sn), a high-densitylithium-containing complex oxide with high stability of a crystalstructure and satisfactory reversibility of charging/discharging can beprovided.

Furthermore, due to the use of the above-mentioned lithium-containingcomplex oxide as a positive active material, a non-aqueous secondarybattery with a high capacity excellent in durability can be provided.The above-mentioned lithium-containing complex oxide contains Mn, whichis rich in resource and less expensive, as one of main components.Therefore, such an oxide is suitable for mass-production, and cancontribute to the reduction in cost.

1.-17. (canceled)
 18. A method for producing a lithium-containingcomplex oxide, comprising: mixing a complex compound containing at leastNi and Mn as constituent elements and a compound of Li at apredetermined ratio; preliminarily heating the mixture at 250° C. to850° C. for 0.5 to 30 hours; and sintering the mixture at a sinteringtemperature higher than a temperature during the preliminary heating,thereby forming a complex oxide with a layered configuration that is thesame as that of LiNiO₂.
 19. The method for producing alithium-containing complex oxide according to claim 18, wherein thesintering temperature is in a range of 700° C. to 1100° C.
 20. Themethod for producing a lithium-containing complex oxide according toclaim 18, wherein, for mixing the complex compound containing at leastNi and Mn as constituent elements and the compound of Li at apredetermined ratio, the complex compound and the compound of Li aredispersed in a solvent to form a slurry.
 21. The method for producing alithium-containing complex oxide according to claim 18, wherein thesintering is performed in an atmosphere containing oxygen.
 22. Themethod for producing a lithium-containing complex oxide according toclaim 18, wherein the lithium-containing complex oxide has a compositionrepresented by General Formula: Li_(1+x+α)Ni_((1−x+δ)/2)Mn_((1−x−δ)/2)O₂(where 0≦x≦0.05, −0.05≦x+α≦0.05, −0.1≦δ≦0.1).
 23. The method forproducing a lithium-containing complex oxide according to claim 18,wherein an average valence of Mn is 3.3 to
 4. 24. A method for producinga lithium-containing complex oxide according to claim 23, wherein anaverage valence of Mn is about
 4. 25. The method for producing alithium-containing complex oxide according to claim 18, wherein anaverage valence of Ni is about
 2. 26. The method for producing alithium-containing complex oxide according to claim 18, wherein thecomplex compound is one selected from an oxide and a hydroxide.
 27. Amethod for producing a lithium-containing complex oxide, comprising:mixing a complex compound containing at least Ni and Mn as constituentelements and a compound of Li at a predetermined ratio; preliminarilyheating the mixture at 250° C. to 850° C. in an atmosphere containingoxygen; and sintering the mixture at a sintering temperature higher thana temperature during the preliminary heating, thereby forming a complexoxide with a layered configuration that is the same as that of LiNiO₂.28. The method for producing a lithium-containing complex oxideaccording to claim 27, wherein the sintering temperature is in a rangeof 700° C. to 1100° C.
 29. The method for producing a lithium-containingcomplex oxide according to claim 27, wherein, for mixing the complexcompound containing at least Ni and Mn as constituent elements and thecompound of Li at a predetermined ratio, the complex compound and thecompound of Li are dispersed in a solvent to form a slurry.
 30. Themethod for producing a lithium-containing complex oxide according toclaim 27, wherein the sintering is performed in an atmosphere containingoxygen.
 31. The method for producing a lithium-containing complex oxideaccording to claim 27, wherein the lithium-containing complex oxide hasa composition represented by General Formula:Li_(1+x+α)Ni_((1−x+δ)/2)Mn_((1−x−δ)/2)O₂ (where 0≦x≦0.05,−0.05≦x+α≦0.05, −0.1≦δ≦0.1).
 32. The method for producing alithium-containing complex oxide according to claim 27, wherein anaverage valence of Mn is 3.3 to
 4. 33. A method for producing alithium-containing complex oxide according to claim 32, wherein anaverage valence of Mn is about
 4. 34. The method for producing alithium-containing complex oxide according to claim 27, wherein anaverage valence of Ni is about
 2. 35. The method for producing alithium-containing complex oxide according to claim 27, wherein thecomplex compound is one selected from an oxide and a hydroxide.
 36. Amethod for producing a lithium-containing complex oxide, comprising:mixing a complex compound containing at least Ni, Mn and M asconstituent elements (where M is at least one element selected from thegroup consisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge and Sn) and acompound of Li at a predetermined ratio; preliminarily heating themixture at 250° C. to 850° C. for 0.5 to 30 hours; and sintering themixture at a sintering temperature higher than a temperature during thepreliminary heating, thereby forming a complex oxide with a layeredconfiguration that is the same as that of LiNiO₂.
 37. The method forproducing a lithium-containing complex oxide according to claim 36,wherein the sintering temperature is in a range of 700° C. to 1100° C.38. The method for producing a lithium-containing complex oxideaccording to claim 36, wherein, for mixing the complex compoundcontaining at least Ni, Mn and M as constituent elements and thecompound of Li at a predetermined ratio, the complex compound and thecompound of Li are dispersed in a solvent to form a slurry.
 39. Themethod for producing a lithium-containing complex oxide according toclaim 36, wherein the sintering is performed in an atmosphere containingoxygen.
 40. The method for producing a lithium-containing complex oxideaccording to claim 36, wherein the lithium-containing complex oxide hasa composition represented by General Formula:Li_(1+x+α)Ni_((1−x−y+δ)/2)Mn_((1−x−y−δ)/2)M_(y)O₂ (where 0≦x≦0.05,−0.05≦x+α≦0.05, 0<y≦0.2, −0.1≦δ≦0.1; M is at least one element selectedfrom the group consisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge and Sn). 41.The method for producing a lithium-containing complex oxide according toclaim 36, wherein the lithium-containing complex oxide has a compositionrepresented by General Formula:Li_(1+x+α)Ni_((1−x−y+δ)/2)Mn_((1−x−y−δ)/2)M_(y)O₂ (where 0≦x≦0.05,−0.05≦x+α≦0.05, 0.2<y≦0.4, −0.24≦δ≦0.24; M is at least one elementselected from the group consisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge andSn).
 42. The method for producing a lithium-containing complex oxideaccording to claim 36, wherein an average valence of Mn is 3.3 to
 4. 43.The method for producing a lithium-containing complex oxide according toclaim 42, wherein an average valence of Mn is about
 4. 44. The methodfor producing a lithium-containing complex oxide according to claim 36,wherein an average valence of Ni is about
 2. 45. The method forproducing a lithium-containing complex oxide according to claim 36,wherein the complex compound is one selected from an oxide and ahydroxide.
 46. A method for producing a lithium-containing complexoxide, comprising: mixing a complex compound containing at least Ni, Mnand M as constituent elements (where M is at least one element selectedfrom the group consisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge and Sn) anda compound of Li at a predetermined ratio; preliminarily heating themixture at 250° C. to 850° C. in an atmosphere containing oxygen; andsintering the mixture at a sintering temperature higher than atemperature during the preliminary heating, thereby forming a complexoxide with a layered configuration that is the same as that of LiNiO₂.47. The method for producing a lithium-containing complex oxideaccording to claim 46, wherein the sintering temperature is in a rangeof 700° C. to 1100° C.
 48. The method for producing a lithium-containingcomplex oxide according to claim 46, wherein, for mixing the complexcompound containing at least Ni, Mn and M as constituent elements andthe compound of Li at a predetermined ratio, the complex compound andthe compound of Li are dispersed in a solvent to form a slurry.
 49. Themethod for producing a lithium-containing complex oxide according toclaim 46, wherein the sintering is performed in an atmosphere containingoxygen.
 50. The method for producing a lithium-containing complex oxideaccording to claim 46, wherein the lithium-containing complex oxide hasa composition represented by General Formula:Li_(1+x+α)Ni_((1−x−y+δ)/2)Mn_((1−x−y−δ)/2)M_(y)O₂ (where 0≦x≦0.05,−0.05≦x+α≦0.05, 0<y≦0.2, −0.1≦δ≦0.1; M is at least one element selectedfrom the group consisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge and Sn). 51.The method for producing a lithium-containing complex oxide according toclaim 46, wherein the lithium-containing complex oxide has a compositionrepresented by General Formula:Li_(1+x+α)Ni_((1−x−y+δ)/2)Mn_((1−x−y−δ)/2)M_(y)O₂ (where 0≦x≦0.05,−0.05≦x+α≦0.05, 0.2<y≦0.4, −0.24≦δ≦0.24; M is at least one elementselected from the group consisting of Ti, Cr, Fe, Co, Cu, Zn, Al, Ge andSn).
 52. The method for producing a lithium-containing complex oxideaccording to claim 46, wherein an average valence of Mn is 3.3 to
 4. 53.The method for producing a lithium-containing complex oxide according toclaim 52, wherein an average valence of Mn is about
 4. 54. The methodfor producing a lithium-containing complex oxide according to claim 46,wherein an average valence of Ni is about
 2. 55. The method forproducing a lithium-containing complex oxide according to claim 46,wherein the complex compound is one selected from an oxide and ahydroxide.